The laser technique can introduce higher quality and precision to value chains and is an important means to promote the upgrade of the industrial structure of the manufacturing industry. In the laser application field, optical fibre lasers characterized by high conversion efficiency, good heat dissipation performance and stability and the like have become one of the mainstream lasers. Similar to other diode pumped lasers, the optical fibre lasers adopt pump light to achieve a high power density in optical fibres, so that laser-level population inversion of laser operating substances is generated, and oscillatory laser output can be achieved by properly adding a positive feedback circuit (constituting a resonant cavity). The optical fibre lasers essentially convert low-quality pump laser light into high-quality laser light output, and the high-quality laser light output can be applied to various fields such as the medical field, the material processing field and the laser weapon field.
At present, the rare earth doped optical fibre serving as the core component of the optical fibre lasers is the key factor determining the laser characteristic of the optical fibre lasers. Rare earth doped optical fibre preforms are prepared mainly through the porous layer liquid-phase doping method, the sol-gel method, the high-temperature flashing method, and the direct nanoparticle deposition method. However, all existing techniques cannot overcome the defects of poor doping uniformity of rare earth particles in optical fibres, high impurity content and the like. In addition, due to process limitations, the core diameter of optical fibre preforms and the doping concentration of rare earth ions are at a low level, and consequentially, the cost of laser optical fibres is high; and meanwhile, usage and system debugging are more difficult, and the requirements for commercialization and industrialization of optical fibre lasers cannot be met.
In the prior art, the rare earth doped optical fibre preforms are prepared mainly through the porous layer liquid-phase doping method, the sol-gel method, the high-temperature flashing method rarely used for experimental studies, and the direct nanoparticle deposition method. As for the sol-gel method and the high-temperature flashing method, equipment requirements are high, and the process is complex. The direct nanoparticle deposition method has high raw material requirements and is not beneficial to large-scale preparation. As for the porous layer liquid-phase doping method which is most widely used currently, a porous core layer is deposited on the inner surface of a quartz deposition tube and then is soaked in a solution containing rare earth elements so that the rare earth elements in the solution can be adsorbed into pores of the porous core layer; afterwards, the deposition tube is disposed on a sintering device to dry the porous core layer with inert gases, and the porous core layer is then sintered into a glass layer; and finally, the deposition tube is collapsed into a solid preform.
The porous layer liquid-phase doping method is complex in process, repeated deposition is needed to obtain a doped fiber core structure meeting drawing requirements, and as the doped fiber core structure has to be taken out of an airtight device between the deposition, soaking and drying procedures, impurities are likely to be introduced, and the laser performance of the rare earth doped optical fibre core is affected; and meanwhile, the local soaking effect of the liquid-phase doping method is inconsistent so that the problem of nonuniform longitudinal doping of preforms cannot be solved, and the longitudinal absorption coefficient of rare earth doped optical fibres is inconsistent, which is not beneficial to nonlinear effect control and batch application. Since the four methods mentioned above are all limited by the internal space of reaction tubes, prepared rare earth doped optical fibre cores are small, the number of optical fibres drawn in each batch is limited, and consequentially, the unit cost of optical fibres is high, and the requirement of the laser market for high-uniformity rare earth doped optical fibres cannot be met.
In the optical communication field, a large number of devices are externally provided with metal ion doped optical fibres, such as high-attenuation optical fibres used as the key materials of optoelectronic devices such as optical fibre attenuators. Similar to the laser optical fibre technique, the high-attenuation optical fibre doping technique has been mastered only by Corative Company in Canada and OptoNet Company in Korea internationally at present. The high-attenuation optical fibre doping technique is based on the porous layer liquid-phase doping method and has the problems of poor doping uniformity and low production efficiency caused by dimension and process limitations of optical fibre preforms.